Some input/output interfaces require operation at multiple voltages. For example, an I2S interface can operate at either 3.3 V or 1.8 V, and Intel's HD-Audio specification allows for operation at 3.3 V and 1.5 V. Such interfaces must be tolerant of the higher voltages, which makes them difficult to operate at the lower voltage. In a CMOS design, this is because high-voltage transistors (suited for 3.3 V operation) have a large voltage threshold. When used with low voltage, the transistors are operated with very little overdrive, reducing their current drive, which reduces the speed of the circuit.